1. Field of the Invention
The present invention relates to a structure of an optical interferometer in a receiver, and a controlling method thereof with respect to an optical transmission system of a phase shift keying scheme such as a differential M (M=2n (n is a natural number)) phase shift keying (for example, differential quadrature phase shift keying (DQPSK)) scheme.
2. Description of the Related Art
In recent years, the capacity of an optical communication system has been increased rapidly. However, a keying technique which becomes mainstream remains a binary amplitude shift keying (which is also called on-off keying (OOK)) in a non return-to-zero (NRZ) or return-to-zero (RZ) format. Recently, a keying/demodulation technique such as a duobinary scheme, carrier-suppressed return-to-zero (CSRZ), or differential phase shift keying (DPSK) is utilized in optical communications. In the DPSK, information is carried based on a phase change between two symbols adjacent to each other. In binary DPSK, the phase change is limited to “0” or “p”. A scheme based on four phase changes (0, p/2, p, and 3p/2) is called DQPSK. As compared with conventional OOK, the DPSK obtains an improved optical S/N ratio (optical signal-to-noise ratio (OSNR)) gain of approximately 3 dB, and a tolerance to a non-linear effect. In optical DQPSK, four-value symbols are transmitted, so spectral efficiency can be doubled. In other words, this is a scheme for simultaneously transmitting two digital signals whose phases are modulated, based on signal light having a single frequency. In this scheme, a pulse repetition rate (for example, 20 Gbaud) is half a transmission data rate (for example, 40 Gbit/s). Therefore, a signal spectral width becomes half compared with a conventional NRZ keying scheme or the like. Thus, the requirement to a speed of an electrical device, the adjustment of light dispersion, and polarization mode dispersion are reduced. That is, the optical DQPSK is a promising candidate for a next-generation optical communication system.
A typical optical DQPSK receiver includes a pair of Mach-Zehnder interferometers corresponding to two branches (here A-branch and B-branch) (see, for example, Non-Patent document 1). Each of the Mach-Zehnder interferometers includes two arms. One of the arms has an optical delay element t corresponding to a symbol time in a transmission system. For example, an optical phase difference between the arms of the interferometer is set to “p/4” in the A-branch and set to “−p/4” in the B-branch.
Two output terminals of each of the Mach-Zehnder interferometers are connected with a balanced photo detector for reproducing transmitted data. Note that a structure and an operation of optical DQPSK transmitter/receiver are described in, for example, Patent document 1.
In the optical DQPSK receiver, it is very important that the optical phase difference between the arms of the interferometer is accurately set to “p/4” and “−p/4”. Otherwise, a deterioration occurs in optical S/N ratio which exceeds an allowable range. Here, a delay interferometer such as the Mach-Zehnder interferometer is a filter whose transmission characteristic is periodic. A transmission period of the delay interferometer is called a free spectral range (FSR).
When the amount of phase between the arms is shifted from p/4 (or −p/4) by physical characteristics of the delay interferometer, a temporal change in set temperature, a change of a signal light wavelength, or the like, there is a problem in that a received waveform deteriorates to reduce a code error rate. Therefore, it is necessary to provide a mechanism for continuously monitoring the amount of phase p/4 (or −p/4) and canceling a shift by feedback control when the amount of phase is shifted therefrom. The amount of phase is adjusted by controlling a temperature of a part of the interferometer by using a heater.                [Patent document 1] JP 2004-516743 A (WO 2002/051041, US 2004/008147)        [Non-Patent document 1] “Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission” by R. A. Griffin et al, Optical Fiber Communication Conference and Exhibit, 2002. OFC2002 17-22 Mar. 2002 Pages 367-368        [Patent document 2] JP 2001-217443 A        
When the phase is adjusted (for example, changed from 0 to 2 p) by using only the heater, power consumption becomes larger. On the other hand, in a case of control using only temperature control means such as a Peltier element, it is difficult to finely adjust the phase, so a penalty is caused by the shift from an optimum point. The interferometer has wavelength dependence, so it is necessary to suppress polarization dependence. The wavelength dependence is caused by the influence of external stress on a planar lightwave circuit (PLC) included in the interferometer. Even in a case where a waveguide is produced without distortion during a manufacturing process, when the PLC is heated at the time of control, the external stress is applied to the PLC. Therefore, the waveguide is distorted, so polarization wavelength dependence occurs. When the wavelength characteristic of the free spectral range (FSR) of the delay interferometer has the polarization dependence, the FSR from the point of view of an optical signal is changed depending on a state of an input polarization to the interferometer. A shift of the FSR which is caused by the polarization dependence of the wavelength characteristic of the interferometer is called a polarization dependent frequency (PDF). In a case where the optimum point (for example, p/4 (or −p/4) between optical signal wavelengths of two signals) in the interferometer is set in a polarization state, when different polarization states are incident on the interferometer, the shift from the optimum point occurs. Thus, when the optical signals are demodulated, waveforms thereof deteriorate.
FIG. 1 is a graph showing a relationship between a phase error and a Q penalty. FIG. 1 shows the Q penalty in a case where the phase error is shifted from p/4 as a reference. For example, when it is shifted from p/4 by 6 degrees, the penalty of 1 dB occurs. For example, assuming that the FSR is 21.5 GHz, when the penalty of 0.15 dB occurs when the PDF is 0.06 GHz, and the penalty of 1 dB occurs when the PDF is 0.36 GHz. In a case of fiber touch or the like, a transmitted polarized wave varies on a millisecond time scale. On the other hand, the phase adjustment of the interferometer is performed by temperature control, so the control remains on a second time scale. Therefore, it is difficult to correct the shift of the optimum phase point due to the variation of the polarized wave by the phase adjustment of the interferometer. Thus, the interferometer having no polarization dependence (PDF Polarization Dependent Frequency) is required.